Chronic infection of the lung by Pseudomonas aeruginosa is the major cause of morbidity and mortality in patients with cystic fibrosis. A number of hypotheses have been proposed to explain the predisposition of these patients to prolonged colonization by this microorganism, many of which imply defective defense mechanisms of the airway epithelium. Matrilysin, a matrix metalloproteinase (MMP), functions in innate immunity and facilitates re-epithelialization and neutrophil influx. Matrilysin is markedly expressed in infected lungs and is induced in epithelial cells by exposure to bacteria or bacterial products, specifically P. aeruginosa flagellin. We hypothesize that matrilysin is a key player in respiratory mucosal innate defense by generating an antibacterial activity via proteolysis of a precursor protein. Indeed, preliminary data for this R21 application demonstrates a marked deficiency in non-defensin antibacterial activity in airway fluid from na[unreadable]ve matrilysin-null (Mmp7-/-) mice, and size estimates of this activity suggest that it is a novel factor. Furthermore, this activity is markedly upregulated in wildtype mice in response to P. aeruginosa infection, yet remains deficient in Mmp7-/- lung. To investigate the role of this MMP in lung immunity, we plan to: 1) isolate and characterize the matrilysin- dependent antibacterial in lung;and 2) verify the role of matrilysin in defense against P. aeruginosa infection. These studies will provide important insight into the role of matrilysin in mucosal defense mechanisms. Infection of the lung by various pathogens (e.g., Staphylococcus aureus, haemophilus influenzae, Pseudomonas aeruginosa, and other) are a major cause of community-acquired infection and morbidity and mortality in cystic fibrosis and ventilated patients. We hypothesize that matrilysin is a key player in respiratory mucosal innate defense by generating an antibacterial activity via proteolysis of a precursor protein. These studies will provide important insight into the role of matrilysin in mucosal defense mechanisms.